1. Field of the Invention
The present invention relates to a method of forming a single crystalline magnesia spinel film in which a single crystalline magnesia spinel film is formed on a single crystalline silicon substrate by a vapor-phase epitaxial growth method.
2. Description of the Prior Art
In general, a SOI [silicon on insulator] film obtained by forming a single crystalline insulating film, such as a single crystalline magnesia spinel film, on a single crystalline silicon substrate and further forming a single crystalline silicon film on the single crystalline insulating film is watched with interest as an element material for giving a high integration degree to an integrated circuit, making the higher operation speed and reducing an electric power consumption.
By the way, magnesia spinel is expressed by a chemical formula of MgO.Al.sub.2 O.sub.3 and then a compositional ratio R of magnesium [Mg] is expressed as R=33.3% [=Mg/(Mg+Al)%] in the case where oxygen [O] is not taken into consideration, and magnesia spinel, in which the compositional ratio R of Mg is a stoichiometric compositional ratio as described above, shows the smallest difference from silicon [Si] in a lattice constant and is superior in the thermal stability, so that it has been applied to the single crystalline insulating film in the SOI.
For, an example of an art of the SOI using the single crystalline magnesia spinel film having a nearly stoichiometric compositional ratio, there has been disclosed "a growth technique of a Si/MgO.Al.sub.2 O.sub.3 /Si structure" on page 21 to 26, ED 86-65, Bulletin of Japanese Society of Electronic Correspondence (Ihara). According to this report, a MgO. Al.sub.2 O.sub.3 film having a stoichiometric compositional ratio and a thickness of about 4,000 .ANG. is epitaxially grown on a Si substrate and subjected to a heat treatment at 1,200.degree. C. in an atmosphere of wet N.sub.2 gas to oxidize an interface between the Si substrate and the MgO. Al.sub.2 O.sub.3 film, whereby forming a SiO.sub.2 layer, and then a single crystalline Si film having a thickness of about 3,000 .ANG. is epitaxially grown on the MgO. Al.sub.2 O.sub.3 film to dope the single crystalline Si film at a carrier concentration n=1.times.10.sup.17 cm.sup.-3, whereby an electron hole mobility amounts to 500 cm.sup.2 V.sec.
In addition, it has been reported that a drain leak current I.sub.D amounts to 10.sup.-11 A/micron when n-MOSFET was formed on the single crystalline Si film with a ring type pattern having W/L=1130/10 microns.
However, in the case of the SOI film using MgO.Al.sub.2 O.sub.3 with the stoichiometric compositional ratio, as described in the literature, the drain leak current of MOSFET is dependent upon a concentration of atoms of Mg and Al taken in the Si film by the reaction of Si upon MgO.Al.sub.2 O.sub.3 in the initial growth stage of the single crystalline Si film, which is an upper layer, so that a sufficiently low value of drain leak current can not be obtained. Also the electron hole mobility is inferior to the case of the SOI film produced by the laser recrystallization method. Accordingly, the conventional SOI film obtained by epitaxially growing a single crystalline MgO.Al.sub.2 O.sub.3 film maintaining a nearly stoichiometric compositional ratio on a single crystalline Si substrate and epitaxially growing a single crystalline Si film on the upper layer has a problem that the characteristics of the resulting device can not be highly improved.